This is why cable is dying. Networks treat praised shows like garbage, putting them in the Friday night death slot, and relying on the archaic Neilsen ratings for all of their shows while ignoring the fact that people lap up them legally online whenever possible. I don't have cable at my tv, so I watched Book 1 and 2 from the Nick site and Book 3 from the upstairs DVR. It didn't matter either way I watched the show; the ads would be muted/ignored/insulted with foul language anyway, but the advertisers "got they're message out".
Nick has been especially on edge about this one. Female lead? "Might not work with the young boys." Older cast? "Might not work younger audience in general?" Ratings down from a slightly underwhelming second season and first six episodes leaked online in Spanish from the Latin American Nick site? "Just do 2 every week until it's over, so we can get back to being called the 24 hour Spongebob channel."
I really hope book 4 really is in the works and Nick doesn't try to weasel there way out of it. The only good I think can come of this is finding out Nick decided Korra does have a big fanbase that just watches it online instead of 7pm on Friday and decided the best direction to take it without canceling it outright was solely online. Maybe that will set a precedent for other networks like Fox to not cancel that Friday night cult show just because the latest Neilsen sweep showed low numbers, when the show's real potential is being available online.